This invention relates generally to integrated circuits and, more particularly, to an integrated circuit such as an operational amplifier wherein a parameter of the circuit such as input offset voltage is trimmed by metal migration.
The term "trimming" as used herein means the fine adjustment of parameters such as resistance, capacitance, or inductance in an integrated circuit. Also, as used herein, the term "metal migration" refers to the movement of metal into a semiconductor crystal such as a resistor by passing current pulses of high amplitude and generally small pulse width through the resistor.
As is well known, it is often times necessary to "match" (i.e. make electrically identical) resistors, transistors, diodes, etc. or to set the absolute value of a single electronic device to a certain value (i.e. trimming a resistor to establish a desired current level). This is done, for example, to minimize the input offset voltage of an operational amplifier.
Resistor trimming is, by far, the most common method of adjusting electrical parameters on integrated circuits. Two methods are generally known. The first utilizes a trim potentiometer external to the semiconductor device but connected to it through pins (e.g. on the printed circuit board). The second method involves trimming the resistor on an integrated circuit die itself, usually while in wafer form.
Resistors may be trimmed mechanically by means of abrasion, usually sand blasting, and laser shaping of a conductive layer. Another approach is to blow fuses which involves the vaporization of metal by passing high current therethrough. Zener zapping involves the short circuiting of diodes by means of excessive current. In addition, the resistance of resistors may be altered chemically as by anodizing.
Known methods of resistor trimming on the die have serious disadvantages. Laser trimming equipment is very expensive and its maintenance and programming extremely critical and costly. The blowing of fuses can result in service contamination and cracks in the protective glass layer which, in turn, may cause poor reliability. Zener zapping requires dedicated circuitry which may amount to over 20% of the die area.
A further disadvantage in the well known methods of resistor trimming on the die is characterized in that after the die is trimmed (usually while in wafer form) it is scribed and assembled in a package. The assembly process places stresses on the semiconductor chip which can cause the trim resistor to change in value due to piezoresistive effects.
It is now known that a specially designed diffused resistor on an integrated circuit die may be trimmed by metal migration. This is accomplished by pulsing the resistor with high amplitude, small duty cycle current pulses. This approach is especially useful in production and has definite advantages over laser trimming, link blowing, or zener zapping. First, the resistors can now be trimmed after the integrated circuit is packaged. Second, the resistor trimmed by metal migration (RTMM) occupies very little die area. Third, when using RTMM resistors, a test computer performs a trim algorithm, not digital logic on the chip. Fourth, RTMM resistors exhibit increased accuracy and resolution; i.e. they undergo resistance changes as small as 25 milliohms/pulse on a 25 ohm resistor. Finally, no additional expensive equipment is required.
Assuming a silicon substrate and aluminum contacts, when small duty cycle current pulses are applied to the RTMM resistors, electron momentum exchange causes some movement of silicon and aluminum atoms in the direction of electron current flow, commonly referred to as elctromigration. As the resistor temperature rises, however, the ability of aluminum to dissolve silicon increases. This phenomenon causes an aluminum filament to grow from the positive terminal, dissolving silicon as it grows, towards the negative terminal, against the flow of electrons. As the filament grows, the resistive value decreases.
It would therefore be desirable to provide circuitry which is configured so as to incorporate the desirable features of RTMM resistors.